There is no spine, but the body is very huge, mainly relying on the buoyancy of water to support its huge body.

The shape of the body is like a transparent umbrella, and the diameter of the umbrella is large and small. The umbrella of the big jellyfish can reach 2 meters in diameter. There are some whisker-like strips on the edge of the umbrella, which are called tentacles. Some tentacles can be as long as 20 to 30 meters, which is equivalent to the length of a big whale.

The tentacles are covered with stinging cells, like small beans stuck to the tentacles. This stinging cell can shoot poisonous silk, and when it meets an "enemy" or prey, it will shoot poisonous silk to scare off the "enemy" or poison it.

There is a small ball on the thin handle in the middle of the tentacle, and there is a small "listening stone" in it, which is the "ear" of the jellyfish.

The water content of jellyfish reaches 98%, and its eating, digestion and excretion must be completed in water. Without water, jellyfish's body will become smaller and uglier.

Morphological characteristics:

Jellyfish usually live a single, floating or swimming life. Few species are in groups, and some groups can live a fixed life. Jellyfish are bell-shaped or inverted bowl-shaped, or umbrella-shaped. The outward protruding side is called the outer canopy or the upper canopy, and the concave side is called the lower canopy.

There is a hanging tube called handle in the center of the lower umbrella surface. The free end of the handle is the mouth, and there is a circle of tentacles around the umbrella. The edge of the lower umbrella of Lepidoptera protrudes inward, and there is a narrow membranous structure called sail membrane, which is the characteristic of hydra. The jellyfish in the pot have no membrane.

The body wall structure of jellyfish type is basically similar to that of hydra type, and it is also composed of two layers of epithelial muscle cells sandwiched between mesothelium, but the mesothelium of jellyfish type is far more developed than that of hydra type.

The gastric circulation cavity surrounded by the body wall is also relatively developed, which is either a simple sac or divided into four gastric sacs by the membrane. Four (hydra jellyfish) or more (bowl jellyfish) radiation tubes extend from the gastric sacs to the umbrella rim, and are connected with annular tubes parallel to the umbrella rim. Centrifugal tubules can also extend from the umbrella rim.

At the base of an umbrella or tentacle, there are sensory organs, such as eye points or foot sacs.